
Eddy's method (and all guide number systems) relies on the fact that light per unit area increases and decreases as a function of the distance squared. If you double the flash to subject distance, you get 1/4 the original light per square unit of area. If you cut the distance in half, you get 4 times the amount of light per square unit of area. That's two stops of adjustment for doubling or cutting the distance in half. For 4 times the distance, or 1/4 the flash to subject distance you need to adjust by 4^2 or 16 times (or 1/16), which is 4 stops of adjustment in the appropriate direction. So with your 1 meter reference settings, you should be able to work that out.
Remember that it's the flash to subject distance that counts, not film or lens to subject. This can make a difference at macro distances. You'll also need to account for light "loss" from extra lens extension at macro distances. George Lepp and John Shaw (esp. Closeups in Nature) have published useful hints on using flash for macro work.
Lee
Last edited by Lee L; 06022007 at 06:58 AM. Click to view previous post history.